22 replies to this topic

[racingbob]

my mate has just put some of the 1275 pressed steel rockers on his mpi

 

to see if he notices any difference

 

and he reckons he notices a slight difference in performance

 

seems to open another 2mm

 

any comments

[Cooperman]

Very doubtful there will be any noticeable difference on a road car. It's all in the mind  .

[racingbob]

Very doubtful there will be any noticeable difference on a road car. It's all in the mind  .

he has got a modified head and maniflow

[Cooperman]

Still won't make much, if any, difference. Even 1.5:1 roller tip rockers only give around 2 bhp extra and that is at very high revs with no gain below around 5500 rpm.

[Spider]

If the valves are opening another 2 mm, then yeah, you would notice a difference.

 

Rather surprising though that there's that much variation between Pressed and Sintered types. I've not checked the Sintered ones to see what the true ratio of them is.

[racingbob]

If the valves are opening another 2 mm, then yeah, you would notice a difference.

 

Rather surprising though that there's that much variation between Pressed and Sintered types. I've not checked the Sintered ones to see what the true ratio of them is.

Chris he said sintered was 40mm and pressed 42mm when measuring them on the head

so I assume 2mm more open, he took it for a spin after by himself

and came back and was sure it had more poke

[Cooperman]

I was really surprised at how little measured difference going from sintered 1.3:1-ish to 1.5:1 R-R's made. It put the bhp up by 2 bhp at 5700 and over, whilst losing a bit at the bottom end. We didn't have time to measure the torque change, but I reckon I lost a bit low down.

 

I make the actual increased lift between 1.25:1 & 1.3:1 to be around 12 thou (0.012") = 0.3 mm. Going from 1.25:1 to 1.5:1 will give an increase in maximum lift of .060" which is 1.5 mm.

 

Can't see where a gain of 2 mm came from unless he has fitted 1.7:1 ratio rockers.

[racingbob]

I was really surprised at how little measured difference going from sintered 1.3:1-ish to 1.5:1 R-R's made. It put the bhp up by 2 bhp at 5700 and over, whilst losing a bit at the bottom end. We didn't have time to measure the torque change, but I reckon I lost a bit low down.

 

I make the actual increased lift between 1.25:1 & 1.3:1 to be around 12 thou (0.012") = 0.3 mm. Going from 1.25:1 to 1.5:1 will give an increase in maximum lift of .060" which is 1.5 mm.

 

Can't see where a gain of 2 mm came from unless he has fitted 1.7:1 ratio rockers.

I don't know weren't 1.7 just the ordinary early pressed

he said was definitely 2mm total difference

 

I will have to take it for a drive see how it goes

have driven it before, he said it pulled better

[Cooperman]

The original were the presses steel type which were stated as 1.3:1. The sintered ones are claimed to also be 1.3:1 but experience has shown them to be around 1.25:1.

You can buy a 1.5:1 roller-tip version, but they are expensive for what you gain. The 1.7:1 are not widely used, except in racing as they tend to cause more rapid cam lobe wear, for obvious reasons.

It sounds as though the measurement of an additional 2 mm is incorrect if it is a case of swapping standard sintered rockers for early presses type.

Take the valve lift as 0.300" and do the maths. 0.3/1.25 x 1.3 = 0.312", so an increase in lift of 0.012" (= 0.3 mm). Maybe he measured 0.2 mm not 2 mm.

The performance gain when going from sintered to pressed types is the reduced inertia of the pressed type, especially when using very high revs. There will be no noticeable gain by lifting the valves an extra 0.012" at full lift.

[Fast Ivan]

maybe the perceived performance gain is due to the recent setting of the valve clearances when the rockers were changed, so its basically running a bit better?

just a thought

[nicklouse]

I feel someone is useing a special means to measure things.

I could never ever feel any difference between sintered.pressed or forged.

Now one set might have a very worn pad?

And did they not do some offset bushed kits in the 80s or some such.

[Dan]

Maybe his original sintered rockers were worn out.

[racingbob]

Maybe his original sintered rockers were worn out.

both sets of rockers are in good nik

 

will have to find out more about this

 

he set tappets to 12 same as before

[nicklouse]

some reading.

 

http://www.calverst...._compendium.htm

[Dusky]

Maybe the difference is because of the modern manifacturing tolerances? EG: previous 'old' rockers were all like 1.25, 1.23,1.26,...

WHile the new are all 1.3? Could give  a VERY slight better 'balance'? ( just a thought, don't know if this is true)